How to use linq `Except` with multiple properties with different class?

Question

I am trying to learn the Linq/Lambda expressions and was stuck at somewhere.

What I was Doing

I have created two classes with

Answer 1

Public Class dosyalar
    Public adi As String
    Public tarih As DateTime
End Class


Private Sub Button2_Click(sender As Object, e As EventArgs) Handles Button2.Click
    Dim dosyalarList1 As New List(Of dosyalar)
    dosyalarList1.Add(New dosyalar With {.adi = "dosya1", .tarih = New DateTime(2020, 3, 30, 10, 10, 10)})
    dosyalarList1.Add(New dosyalar With {.adi = "dosya2", .tarih = New DateTime(2020, 3, 30, 10, 10, 20)})
    dosyalarList1.Add(New dosyalar With {.adi = "dosya3", .tarih = New DateTime(2020, 3, 30, 10, 10, 30)})
    dosyalarList1.Add(New dosyalar With {.adi = "dosya4", .tarih = New DateTime(2020, 3, 30, 10, 10, 40)})
    dosyalarList1.Add(New dosyalar With {.adi = "dosya5", .tarih = New DateTime(2020, 3, 30, 10, 10, 50)})

    Dim dosyalarList2 As New List(Of dosyalar)
    dosyalarList2.Add(New dosyalar With {.adi = "dosya1", .tarih = New DateTime(2020, 3, 30, 10, 10, 11)})
    dosyalarList2.Add(New dosyalar With {.adi = "dosya2", .tarih = New DateTime(2020, 3, 30, 10, 10, 21)})
    dosyalarList2.Add(New dosyalar With {.adi = "dosya3", .tarih = New DateTime(2020, 3, 30, 10, 10, 30)})
    dosyalarList2.Add(New dosyalar With {.adi = "dosya4", .tarih = New DateTime(2020, 3, 30, 10, 10, 40)})
    dosyalarList2.Add(New dosyalar With {.adi = "dosya5", .tarih = New DateTime(2020, 3, 30, 10, 10, 50)})


    Dim result As List(Of dosyalar) = dosyalarList1.Where(Function(a) Not dosyalarList2.Any(Function(b) a.adi = b.adi AndAlso a.tarih = b.tarih)).ToList

End Sub

Answer 2

You have said that you need only these objects from testListA:

• there is not matching ProductID in testListB
• there is existing mathing ProductID, but with different Category

So, your filter must be:

!testListB.Any(b => a.ProductID == b.ProductID && a.Category == b.Category)

So, change your code as:

testListA.Where(a => !testListB.Any(b => a.ProductID == b.ProductID && a.Category == b.Category));

---

Second approach:

Or you can create a new List<TestA> from the second list:

 var secondListA = testListB.Select(x=> new TestA(){Category=x.Category, ProductID=x.ProductID}).ToList();

And then create your Comparer:

sealed class MyComparer : IEqualityComparer<TestA>
{
    public bool Equals(TestA x, TestA y)
    {
        if (x == null)
            return y == null;
        else if (y == null)
            return false;
        else
            return x.ProductID == y.ProductID && x.Category == y.Category;
    }

    public int GetHashCode(TestA obj)
    {
        return obj.ProductID.GetHashCode();
    }
}

And use Except() overload which produces the set difference of two sequences by using the specified IEqualityComparer<T> to compare values.:

var result = testListA.Except(secondListA, new MyComparer ()).ToList();

Answer 3

This is an example implementing IEqualityComparer. It should work as a concept for your purpose:

private class PKExclusiveComparer<T> : IEqualityComparer<T> where T : DataRow
{
  public bool Equals(T x, T y)
  {
    bool result = true;
    foreach (DataColumn col in (x as DataRow).Table.Columns)
    {
      if (!(x as DataRow).Table.PrimaryKey.Contains(col))
      {
        result &= x[col].Equals(y[col]);
      }
    }
    return result;
  }

  public int GetHashCode(T x)
  {
    string code = string.Empty;
    foreach (DataColumn col in (x as DataRow).Table.Columns)
    {
      if (!(x as DataRow).Table.PrimaryKey.Contains(col))
      {
        code += x[col].ToString();
      }
    }
    return code.GetHashCode();
  }
}